Macular degeneration is a leading cause of progressive blindness. The macula is the central region of the retina and contains the fovea where high acuity central vision is processed. Macular degeneration is a neurodegenerative disease in the macula that progressively causes disabling deficits in central vision which is processed by foveal cone photoreceptors, in night vision which is processed by rod photoreceptors, and in dark adaptation under conditions of both daylight (cones) and darkness (rods).
There are multiple forms of macular degeneration. Dry age-related macular degeneration (AMD) is the initial and most common form and first appears at middle age or later. Its clinical signs include an increase in fundus auto-fluorescence (FAF) and the formation of extracellular deposits called soft drusen, both caused by the accumulation of lipofuscin in retinal pigment epithelial (RPE) cells as discussed below. About 40% of dry AMD patients progress to an advanced form of the disease called geographic atrophy (GA), secondary to dry AMD, characterized by one or more atrophic retinal lesions caused by the localized death of RPE cells and adjacent retinal photoreceptor cells. Another 10% of dry AMD patients progress to wet AMD, characterized by neovascular growth from the choroid into the retina which disrupts retinal tissue and thereby destroys visual function. Finally, there is an early onset form of macular degeneration called Stargardt disease, which first appears in teenagers and young adults. Stargardt disease is believed to have the same etiology as dry AMD, but does not involve choroidal neovascularization as it progresses.
Multiple lines of evidence indicate that macular degeneration is caused by the gradual accumulation in RPE cells of a naturally occurring bis-retinoid compound called A2E (Sparrow J. R. et al., Phospholipid meets all-trans-retinal: the making of RPE bisretinoids, J. Lipid Res. Aug. 7, 2009). A2E is a cytotoxic product from the reaction of all-trans retinaldehyde (RAL) and phosphatidylethanolamine (PE), a membrane phospholipid found in the disc membranes of photoreceptor outer segments. The RAL that reacts with PE escapes from the visual cycle (step 3b in FIG. 1), a metabolic pathway in the back of the eye. The visual cycle (i) converts vitamin A from an alcohol (retinol) to a photoreactive aldehyde (11-cis-retinaldehyde) for use in phototransduction by opsin proteins in photoreceptor cell outer segments, and (ii) converts RAL to retinol after photo-transduction. As RAL escapes the visual cycle, A2E precursors form reversibly in photoreceptor outer segments, which are ingested by neighboring RPE cells after diurnal shedding. The final and irreversible step in the biosynthesis of A2E takes place in the acidic environment of RPE cell lysosomes.
As A2E accumulates in RPE cells, it gradually poisons them by multiple mechanisms including lysosomal failure and oxidative stress. Lysosomal failure leads to the accumulation of undigested cellular debris called lipofuscin, which contains A2E and can be detected clinically by FAF imaging. Oxidative stress leads to RPE cell death by apoptotic mechanisms in GA, and triggers VEGF signaling by RPE cells which causes the choroidal neovascular growth that is the hallmark of wet AMD. Complement cascades are activated by oxidized A2E in drusen and cause further pathology by inflammatory pathways. As RPE cells deteriorate, they lose their ability to participate in the visual cycle and are unable to provide photoreceptors with the metabolic support required for normal visual function. As this metabolic support is withdrawn, photoreceptors fail to renew their shed outer segments and visual function is progressively lost. By reducing the formation of A2E pharmacologically, RPE cells can recover from A2E toxicities and resume their normal metabolic support of photoreceptor cells.
The PCT publication WO 2006/127945 discloses compounds and compositions that have been shown to reduce the formation of A2E. Those compounds are designed to inhibit A2E biosynthesis by reducing the amount of free RAL available for reaction with PE in photoreceptor outer segments. However, there still exists a need for compounds with desirable properties, such as improved potency and/or pharmacological half-life. The present application addresses that need.